Small Angle Coherent Scattering of Gammas by Bound Electrons

Franz has shown that for nonrelativistic changes of photon momentum (q) the coherent scattering of gammas by bound electrons reduces to the form factor calculation. We consider the near-relativistic region, by an expansion in q/mc. The corrections to the form factor calculation are very small for small atomic numbers, the order of magnitude being (Z/137)(q/mc). Numerical results for scattering by K electrons of tin give an increase of the amplitude of 25 percent above the form factor value for q/mc=1.5 and a smaller change for smaller values of q. Our results are incomplete since we neglect the effects of binding in the intermediate state.